The use of studded tires has been prohibited by law for the purpose of preventing dust pollution by studded tires. This has led to use of studless winter tires in place of studded tires in cold regions.
For improved grip performance on ice and snow of such studless winter tires, a method is known in which the hardness of a rubber used in a tire is reduced so as to reduce the elastic modulus (or modulus of elasticity) at low temperatures and thus enhance the adhesive friction. In particular, since the grip performance on ice is highly affected by the effective contact area between the rubber and ice, a flexible rubber is required for a larger effective contact area. However, rubbers lose their flexibility and become hard over time. This means that a flexible rubber obtained only by decreasing the hardness cannot maintain its good grip performance. Thus, there is still room for improvement in this respect.
Petroleum oil added to improve the flexibility of rubbers bleeds out from the compound, which leads to hardening of the compound with time, with the result that good performance on ice and snow cannot be maintained. Thus, there is also room for improvement in this respect. Further, the addition of petroleum oil tends to decrease the abrasion resistance. Thus, there is further room for improvement in this respect.
Recent serious consideration of global environmental issues has led to development of some novel oil alternatives to petroleum oil. For example, Patent Literature 1 discloses rubber compositions containing a vegetable oil or fat such as palm oil. However, although such vegetable oil and fat is better from the viewpoint of environmental friendliness, it is inferior to petroleum oil such as aromatic oil in the dispersibility of filler. Thus, there is further room for improvement in this respect.
Meanwhile, tires are known to deteriorate by heat generated during driving, ozone and oxygen in the air, ultraviolet rays, and the like. In recent years, there has been an increasing trend in ozone levels because of, for example, industrialization. Thus, rubbers are required to have much better ozone resistance to prevent their deterioration and thus provide a longer life to tires.
In order to improve the ozone resistance, a method is known in which an antioxidant, wax or the like is added. In this method, however, such antioxidant and wax will migrate to the tire surface to discolor the surface, resulting in poor appearance. Such deterioration in appearance can be a disadvantage especially for side walls. Further, it can also be a disadvantage for treads of vehicles that are not driven often and the like.